4. In article 17 (scientific instruments etc), in paragraph (5)(b) after “navigation” insert “, in particular where any such deposit is tethered to the seabed or reduces navigational clearance by more than 5% by reference to Chart Datum(1)”.
Chart Datum is the plane below which all depths are published on a navigational chart. It is also the plane to which all tidal heights are referred, so by adding the tidal height to the charted depth, the true depth of water is determined. By international agreement Chart Datum is defined as a level so low that the tide will not frequently fall below it. In the United Kingdom, this level is normally approximately the level of Lowest Astronomical Tide.